We use the definite article 'the' when the noun is followed by a clause or phrase which identifies it. In the given sentence, the 'doctor' is identified with the clause 'who has a clinic beside my home'. Option C is correct here since we are referring to a specific doctor. Articles 'a' and 'an' are for nouns which are not specific, since the noun here is specific, these options are incorrect. Option D is incorrect because an article is required for the noun.